DE 195 33 976 A1 makes known a hydraulic system for a V-belt, stepless transmission, wherein two hydraulic pressure branches, having two different hydraulic pressures are realized. Connection lines from the first hydraulic pressure branch lead to pressure users, the users being, for instance, corresponding solenoid pressure regulation valves, which are controlled by an electronic control center upon need. In this document, the users will include the primary disk S1, the secondary disk S2, a hydrodynamic torque converter and the lubrication positions for the stepless transmission as well as the vehicle clutch and/or the vehicle operational brake.
The pressure in the first hydraulic branch is adjusted by a controllable pressure regulation valve, which, in the course of maintaining the desired pressure, bypasses to the transmission lubrication storage facility that quantity of pump capacity which is not needed. The pressure regulation function of the said valve, in this operation, is carried out by a separate, solenoid pressure controller. At the same time, this solenoid pressure controller is so regulated, that the pressure in the first hydraulic pressure branch and thus in the line leading to the primary disk as well as to the clutch and/or to the vehicle brake, represents at least, the maximum, necessarily required pressure for their operation.
In the first hydraulic pressure branch, the line forks to a pressure line with a pressure reducing valve, the outlet port of which connects to a pressure line leading to a second hydraulic branch. This pressure reducing valve determines the pressure level of the second hydraulic pressure branch and maintains the pressure constant at that level.
From the constant pressure level which is held in the second hydraulic pressure branch, the solenoid pressure regulator creates a pilot pressure for the adjustment of pressure in the thereto connected lines to various users. This enables, for example, that even the pressure in the hydraulic lines leading to the primary disk is adjustable.
Although it is true, that this hydraulic control system possesses its own advantages, the disadvantage is, that the pressure in the first hydraulic pressure branch is selected, in practice, to be significantly greater than that of the highest necessary pressure level for the remaining hydraulic users. This pressure difference (hereinafter called “offset”) runs some three to five bar, and must be additionally generated by the hydraulic pump. This leads to a lessened degree of efficiency of the transmission, which finally penalizes the transmission to a class of higher fuel consumption.
Therefore, with this background, the purpose of the invention, is to make available a procedure for the control of a known hydraulic control apparatus for an automatic transmission, wherein, with the same range of functions as is the case with the known hydraulic control system, and wherein an improved efficiency is acquired.